Corrosion of structural steel and other metals is a serious problem in construction and utility industry. When exposed to humid and saline environments, especially at elevated temperatures, steel deteriorates. To minimize or reduce the extent of this corrosion, alloys of steel, such as galvanized (zinc coated) compositions, or chrome plated compositions are used. While this approach may solve the problem in the short run, the problem persists when the steel is exposed to the above-mentioned environments over long periods of time. This invention discloses uniquely-suited phosphate-based composite coatings that minimize or reduce the corrosion of steel or other metals and make it unnecessary to use alloys of steel such as galvanized (zinc coated) compositions or chrome plated compositions.
Phosphating to passivate a steel surface is generally known in the steel industry. Typically, well polished steel is immersed in phosphate bath of pH between 4-4.5 containing 2-3 g/L phosphoric acid, 2-3 g/L of ammonium or zinc dihydrogen phosphate as buffer, and a small amount (<0.5 g/L) of oxidizer, to produce an iron phosphate passivation layer. In the process, however, hydrogen gas is liberated by the reaction of elemental iron with water in the extremely acidic environment. This produces a very thin passivation layer that is porous and not abrasion resistant, and as a result, an additional coating is required to make the surface of the passivated steel inaccessible to atmospheric oxygen. This process has, therefore, at least the following disadvantages: (i) an acid immersion bath/tank, which generates sludge as formed by accumulating reaction products—making the bath less effective and creating environmental disposal issues for the sludge and the acidic solution; (ii) oxidizers used in the passivation process produce toxic gases. For example, chlorates produce chlorine, meta nitro benzene sulfonic acid produces nitrous oxide, potassium permanganate presents occupational health risks; (iii) resultant passivation layers are not abrasion resistant, therefore, abrasion resistance must be augmented by additional coating(s).